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Re: Gusset Plate Design for SCBF and OCBF

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For sure if you are designing an SCBF in a high seismic zone you will need to provide calculations for the gusset plates.  However, the design force for the gusset plate is the "capacity" of the brace, not the seismic force determine from your analysis.  Therefore if your bracing system is relatively symmetrical as you go up the structure then the gusset plates should be the same size for a given brace size.  This is also why you would not just use Fy=36 ksi as you need the actual strength of the brace to design the gusset plate.  You have referenced several good documents but if you have not done so yet look closely at the commentary in the AISC Seismic Provisions.  Also go back and review closely the AISC Seismic Provisions as it clearly defines the forces that the brace connections (which includes the gusset plate) are to be designed for and requirements for the assumed member material properties.

Thomas Hunt, S.E.
ABS Consulting

Szuchuan Chang <szuchuan(--nospam--at)>

12/06/2004 07:55 PM

Please respond to

Gusset Plate Design for SCBF and OCBF


During the plan checking/reviewing processes, can you require the
gusset plates in SCBF or OCBF be submitted for 2t –fold line and the
Uniform Force Method AISC Manual of Steel Construction Volume II –
Heavy Bracing Connection- the design?

Assuming a project to use SCBF or OCBF to be the lateral resisting
system that is located in the SF Bay Area and it is also required to
meet the AISC's "Seismic Provisions for Structural Steel Buildings",
May, 2002

From the notes I have, after the Northridge earthquake, Roy Becker
provided the design examples in his "Seismic Design of Special
Concentrically Braced Steel Frame."  The Uniform Force Method was
introduced to be the new way of analyze the gusset plates.  The
"Seismic Behavior and Deign of Gusset Plates", by Abolhassan
Astaneh-Asl of UC Berkeley 1998, was referred in the Seismic
Provisions in design for out-of-plane bucking of braces.  I looked
over the notes of "Seismic Design and the 2002 AISC Seismic
Provisions". by Thomas A. Sabol, of Englekirk & Sabol, that was given
in October 2003, there was not much requirement about the gusset plate
design in his notes.  In §13.3d of the Seismic Provision "The design
of gusset plates shall include consideration of buckling" was the only
detail requirement on gusset plates.  Last week I got the AISC's
announcement of a 2005 seminar called "Seismic Braced Frames-Design
Concepts and Connections" by Rafael Sabelli of DASSE Design.  Looks
like he is going to introduce a new kind of frame called
Buckling-Restrained Braced Frames (BRBF).

Now that the stiffness of gusset is put into considerations and the
loading cases changes from level to level, each of the gussets can
require a separated set of calculations.  Too much strength or not
enough ductility (plates or weldments) are also considered a bad
thing.  The man-hours required to design a gusset plate that meet the
fold-line and UFM can be quite costly.  What are the current positions
of the building departments on requiring these kind of design and

A side question is about Fy used in design.  Is it acceptable to use
Fy --36 ksi WF shape braces knowing that Fy will most likely to be 50
ksi?  An advantage of smaller member is kind of attractive especially
when the axial load is not a governing matter. Is that cheating?  I
wonder what Mr. Charlie Carter of AISC will say about that!



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